1. Field of the Invention
The present invention relates generally to a liquid level indicating system and more particularly to an improved fuel gauge interface circuit for accurate monitoring liquid level variations within a liquid tank.
2. Description of the Relevant Art
For many years, the auto industry has employed fuel gauge systems to measure fuel level in fuel tanks of motor vehicles. A basic fuel gauge system typically includes a fuel level sensor and a visual gauge to provide the driver the status of the fuel level in the motor vehicle. The fuel gauge indicates the fuel level by receiving fuel level information from the fuel level sensor placed within the fuel tank of the motor vehicle. Ideally, the fuel level sensor will correctly measure the fuel level in the tank at all times during the operation of the motor vehicle. Unfortunately, the normal maneuvering of the motor vehicle causes inevitable transient variations of the fuel level, commonly known as fuel sloshes. As a result, many damping arrangements have been incorporated into commercial fuel gauge systems to overcome rapid transient movements of the fuel gauge due to fuel sloshes. A modern fuel gauge system typically adopts an electrical damping circuit although some older prior arts include mechanical damping devices.
Examples of related prior art techniques are described in U.S. Pat. Nos. 5,257,300; 4,991,435; 3,938,117; 3,678,749; 4,174,626 and 4,179,740.
The prior art techniques normally employ RC circuits to provide signals for the fuel gauge system. This type of anti-slosh fuel gauge system is based on a large capacitor and a large resistor in order to generate an extremely large time constant to provide an electrical damping function used to minimize the slosh effect. The problem with this type of fuel gauge system is that the time constant is inconsistent over the gauge range and therefore can not always provide an accurate reading on the fuel level. Moreover, the RC circuit's time constant changes as the circuit ages. Another problem with modern fuel gauge systems is that a large capacitor and a large resistor require a larger sized integrated circuit to process the signal, making them more expensive to manufacture. They also consume more power. With the existing systems, it is virtually impossible to eliminate these problems without major performance loss or cost penalties.